The present invention relates to a gray scale facsimile signal processing method and equipment, and more particularly to such method and equipment for coding a facsimile signal having a gray scale component (hereinafter referred to as a gray scale facsimile signal), obtained by scanning a picture containing a gray scale, such as a photograph, and decoding the coded information.
Known as conventional methods for coding gray scale picture signal in a manner to compress its frequency band are (1) a predictive coding method, (2) a bit plane method, (3) a half-tone display method and (4) a block coding method.
The coding method (1) is one that predicts the gradation of each picture element signal from the gradation of surrounding picture elements and codes the difference between the predicted signal and an actual picture element signal. The number of surrounding picture elements employed for the prediction is usually in the range of between one to ten. In general, the more the surrounding picture elements are used, the higher the accuracy of prediction improves and the more the difference signal approaches zero, while the prediction processing becomes more and more complex. The coding efficiency of this method is not so high, approximately in the range of 1.5 to 2.0 bits/picture element. In addition, an enormous table (memory) is needed for the prediction processing. Conventional binary facsimile signal coding methods are the modified Huffman coding method and the modified Read coding method internationally standardized by CCITT. The prior art predictive coding method is difficult to apply to facsimile equipment employing such international standard coding methods.
The bit plane method (2) is one that renders the amplitude value of the gray scale signal into a PCM code for each picture element signal and encodes binary (1, 0) pictures of bit planes of the same weight by one of the abovementioned international standard coding methods. The coding efficiency of this method is about 2 to 3 bits/picture element, which is lower than those of the other coding methods. Furthermore, the binary signal of the bit plane of the most significant bit becomes identical with a binary facsimile signal which ignores the gray scale, but as the bit plane of the least significant bit is approached, the binary signal gradually becomes a noise and does not match with the abovesaid international standard coding method.
The half-tone display method (3) is one that divides a picture frame into a plurality of blocks, selects the ratio between the number of black picture elements and the number of white picture elements for each block in a manner to obtain the mean gradation of the block and represents the half tone according to the binary definition of the white and black picture elements. According to this method, in the case where a binary signal obtained at the transmitting side is coded by the aforesaid internationally standardized coding method, a gradation signal is transmitted as it is and, at the receiving side, the gray scale is represented in accordance with the density of white and black picture elements of each block. With this method, since the mean gradation is provided for each block in terms of the ratio between the number of white picture elements and the number of black picture elements in the block, the picture quality is impaired. As the binary signal usually differ widely from the binary facsimile signal in statistical property, it is difficult to apply the aforementioned international standard coding methods.
The block coding method (4) is one that is disclosed, for instance, in U.S. Pat. No. 4,205,341 entitled "Picture Signal Coding Apparatus", issued on May 27, 1980. According to this method, a picture frame is divided into a plurality of blocks, in each of which a binary resolution signal is obtained on the basis of the mean gradation of the block. The block is represented by two gradation signals of the mean gradation of picture element signals corresponding to one of the two values of the resolution signal and the mean gradation of picture element signals corresponding to the other and the resolution signal. The coding efficiency of this method is about 1 bit/picture element and is still insufficient. Moreover, this method is not well suitable for use with the aforesaid international standard coding methods.